The invention relates generally to testing equipment and, more specifically, to an improved shock tube.
A shock tube can be employed for determining the effects of a shock wave on a test or target object. The shock wave is generated by the sudden release of high-pressure gas from within the shock tube driver section. This shock wave provides the primary loading on the target. As the shock wave expands out of the discharge end of the driver, a rarefaction wave is generated which travels into and along the driver. The reflections of the initial shock wave off the target and of the rarefaction off the closed end of the driver may interact to yield secondary loading on the target. Depending on the shock tube geometry, the rarefaction wave can yield a period of reduced or negative pressure on the target.
Very large cylindrical shock tubes can be used when the test or target object is large. However, large shock tubes are very expensive to manufacture, install, and operate. Shock tubes utilizing reduced driver diameters with an expansion section to obtain the desired target dimensions have been therefore employed to minimize these costs.
In an ideal free-field simulation, the pressure loading on the test object rises abruptly to a maximum positive value, decays smoothly at a predetermined rate to a zero pressure value, goes negative, and then rises smoothly at a predetermined rate back to a zero pressure. However, the presence of the expansion section increases the magnitude and effect of rarefaction waves on the test object, and can terminate the primary target positive phase loading prematurely. The secondary waves generated by the interactions of the reflected primary shock and the rarefaction wave can also prevent this idealized target loading from being achieved.
The duration and impulse of the pressure waves generated by a shock tube are governed partially by the length of the driver and extension sections. The use of fixed-length driver and extension sections severely limits the range of achievable target load histories. The use of multiple driver and extension sections can overcome this restriction, but this increases the manufacture, installation, and operation costs. In addition, the target load histories that can be achieved are still limited by the available driver and extension section lengths.
There is, therefore, a need for an improved shock tube that allows the rarefaction and secondary shock waves to be controlled such that the idealized target loading can be more closely approximated, and that allows the duration and impulse of the pressure waves to be controlled without the need for multiple driver and extension sections.
These and other needs are met by the present invention, which in accord with one aspect includes a driver section, an expansion section connected to the driver section, an extension section joined to the expansion section, and shock absorbent material. The driver and expansion sections define a cavity and the shock absorbent material is disposed within this cavity. The shock absorbent material can be disposed on the expansion section sidewalls and proximate to the driver section end wall. By placing shock absorbent material at the end of the driver section, the reflection of the initial rarefaction wave and subsequent shock and rarefaction waves can be mitigated from this surface. Also, shock absorbent material placed in the expansion section can mitigate shock wave reflections from the section sidewalls.
In another embodiment of the present invention, the shock tube includes a driver section, an expansion section connected to the driver section, an extension section joined to the expansion section, and one or more active vents disposed over respective holes in the expansion and/or extension sections connected to the cavity defined by these sections. The shock tube can include two or more active vents that are separate from one another or are connected together with a common manifold. The active vents are employed to control the shape of both the positive and negative target loading phases.
In yet another embodiment of the present invention, the shock tube includes a driver section, an expansion section connected to the driver section, and an extension section joined to the expansion section. The length of the gas space within the driver section is adjustable, so that a wide range of effective driver lengths can be achieved. This facilitates control of the shape of both the positive and negative target loading phases.
In still another embodiment, the shock tube includes a driver section, an expansion section connected to the driver section, and an extension section joined to the expansion section. The extension section length is adjustable, so that a wide range of effective extension section lengths can be achieved. This facilitates control of the shape of both the positive and negative target loading phases.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only an exemplary embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.